1. Field of the Invention
This invention relates to stabilized precious metal sensitizing solutions. The stabilized solutions are used to render surfaces of a substrate catalytic to the reception of an electroless metal.
2. Prior Art
The electroless deposition of a metal on either a metallic or non-metallic substrate usually requires pretreatment or sensitization of the substrate to render it catalytic to the reception of such deposit. Various methods have evolved over the years employing sensitizing compositions.
One of the earliest innovations employed a plurality of baths in which the substrate was subjected to a two-step process entailing immersion first in a stannous chloride solution, followed by a second immersion in an acidic palladium chloride solution. More recently, there has been proposed a unitary treating process which employs a colloidal dispersion -- as contrasted to a clear solution -- of precious metal, e.g., palladium and a Group IV polyvalent metal, e.g., tin; see Shipley, U.S. Pat. No. 3,001,920. A substantial improvement was provided with the development of optically clear, true solutions of complexes of precious metal and Group IV metals. See, for example, the PdCl.sub.2.SnCl.sub.2 reaction products and related sensitizers described in Zeblisky, U.S. Pat. No. 3,672,938. These latter are provided both in concentrated form (Zeblisky, U.S. Pat. No. 3,682,671) and in a dilutable dry powder form (Zeblisky, U.S. Pat. No. 3,672,923). All of these patents are incorporated herein by reference to save unnecessarily detailed description of components, ratios, methods of using the same, and the like.
The old plural bath process, while readily effecting sensitization, deposits a flash coating of the precious metal onto metallic substrates. The precious metal so flash coated from the sensitizing bath disrupts the balance of the bath, causing a loss in stability and a change in concentration. To maintain catalytic activity, the frequent addition of the precious metal bearing solution to such baths is required. It is obvious that this increases the cost of the plural bath process. Furthermore, any flash coating of precious metal normally leads to a low grade of adhesion, thereby materially affecting the bond and peel strength of any subsequent electroless metal deposit.
The colloidal unitary bath process of the Shipley patent, on the other hand, while frequently overcoming the problem of flash coating, has a diminished reactivity in the sense that higher concentrations of precious metal are needed and the bath requires longer periods of time for sensitization. In addition, double metal colloids are needed to improve stability. Moreover, because the precious metal is present in colloidal form, dilutable concentrates of such baths cannot be prepared and shipped and stored because of destruction of the colloidal dispersion by flocculation. As is explained in the above-mentioned '671 and '923 patents, concentrated or dry forms of the unitary bath sensitizers are uniquely useful because shipping and storage costs are minimized; and the process solutions are more simply put together (by dilution, for example). A major disadvantage of such solutions is that they tend to be unstable, especially on esposure to air. After a few weeks, under such conditions, precious metal tends to separate from the solution as a metallic film on the surface of the solution and sometimes as a granular precipitate. If the solution in this state is used to sensitize surfaces for subsequent electroless plating by immersing the surfaces in the solution, upon removal, the surface is coated with a layer of the precious metal which cannot easily be removed by rinsing. If electroless is effected without removing the precious metal coating, the plated metal has poor adhesion to the underlying surface.
In U.K. Pat. No. 1,174,851, sensitizing solutions of the unitary type having improved stability are provided by merely adding a water-soluble, hydroxyl group-containing organic compound, as a stabilizer, e.g., an alcohol, glycerine, a sugar, and the like. According to the U.K. patent, the best stabilizers comprise hydroxyl group-containing aromatic compounds, such as resorcinol, catechol, quinol, and the like, because they can be used in somewhat lower concentrations than alcohols, sugars, etc. These water-soluble hydroxy-aromatic compounds, like the others, are merely to be added to the sensitizing solution, after it is prepared, and the sensitizing solution is said to be rendered stable for at least five months. There is no suggestion in the U.K. patent of any need to put the ingredients together in any specific way, and the patent is silent regarding the appearance, and especially the color of the stabilized sensitizing solutions.
Even though the U.K. patent provides a stable, active sensitizing solution, if the hydroxyl group-containing compound is merely added to the sensitizer solution in the concentrations specified, the solutions are rather slow in their sensitizing activity, immersion times of 15 to 20 minutes being usually required. Moreover, after prolonged storage, there is still some tendency for precious metal to precipitate from the solution and to cause the disadvantages enumerated above.
It has now been discovered that if a hydroxyl group-containing aromatic compound of the type described in the U.K. patent is added in a novel way and at the proper time while preparing a soluble complex metal sensitizer of the type disclosed and claimed in the aforesaid Zeblisky patents, there is produced a reaction product which apparently includes the hydroxyl group-containing aromatic compound. Such solutions unexpectedly provide a substantial increase in the speed of sensitization, acting, in some cases, twice as fast as the best of those described in the prior art. Moreover, and particularly with respect to the stabilized sensitizer of U.K. Pat. No. 1,174,851, there is a substantial improvement in stability and a superior product is formed. A different product is formed herein because of color differences as will be described hereinafter, and these are presumably due to the presence of a complex involving the precious metal and the hydroxy aromatic compound. In the case of palladium and resorcinol, for example, a red complex can be formed, and this color has not before been observed with these components.
It is surprising the view of the U.K. patent that all hydroxyl containing compounds are not suitable. For example, the isopropanol specified as a preferred stabilizer therein will not produce the novel reaction product discovered by applicants herein.
Accordingly, it is a primary object of the present invention to provide a new sensitizer composition which is highly active and stable, as well as processes using such a composition to effect the sensitization of a substrate to render it catalytic to the reception of an electroless metal deposit. It is contemplated also to provide such sensitizers in the form of concentrates and dry, dilutable solids, as generally dscribed in the relevant aforementioned Zeblisky patents.
Another object of the present invention is to provide new and useful stabilized compositions which have greater activity as sensitizers and which are very resistant to the effects of contamination. Such compositions provide more stable sensitizers with greater economy in the use of meterials.